Ever have those days where even the thought of exercise seems daunting? If thats the case then living in space may not be for you. The whole business of weightlessness confuses your body, your bladder doesnt realise when its full (which can block the urethral orifice), your eyeballs change shape, and since you no longer need to support your own weight your muscles will lose 20% of their mass within the first week! This last point might be hard to imagine, after all I bet there has been many an occasion when the gym has eluded you for a lot longer than a week, if youre anything like me anyway. But the fact remains that even if the only exercise you manage is walking to and from the fridge, your body is always unconsciously and relentlessly regulating itself in the form of pumping blood through your arteries and veins, maintaining posture, and more or less perfectly balancing the act of your bones shedding and regeneration cycle. Gravity plays an essential role in this regulation. Without it your heart no longer needs to push blood up to your head, because there is no up in space. You no longer maintain any posture because without a ground to sit or stand on there really isnt any need. And when everything is weightless your bones no longer experience much mechanical stress, and when theres no need for your bones to be strong your body stops replenishing them and a kind of space osteoporosis ensues. This isnt so bad so long as you remain in space, but upon return to Earth, you might just find that the burden of gravity is too much for your fragile heart, as it is no longer capable of pumping blood to your extremities, and your bones are no longer capable of supporting your muscles. Its quite likely that these effects could be fatal if nothing was done to counteract them. This is where the exercise comes in, the astronauts living on the ISS need to exercise for at least two hours every single day, only the CrossFit cultists at your local university can compare to that.